SPINAL NERVES. 858 



CHAPTER II. 



SPINAL NERVES. 



We designate as spinal, vertebral, or rachidian n^rves^ those which emanate from 

 the spinal cord, and leave the vertebral canal by the intervertebral foramina to 

 proceed to the various organs. 



They are estimated at 42 or 43 pairs, and are apj)ortioned as follows in the 

 five regions of the spine : cervical, 8 pxirs ; dorsal, 17 pairs ; lumbar, 6 pairs; 

 sacral, 5 pairs ; and coccygeal, 6 to 7 pairs. 



They differ from the cranial nerves in closely resembling each other in the 

 fundamental points of their constitution. All proceed from the lateral aspects 

 of the spinal cord by two orders of roots : one motor, the otlier sensitive. In all, 

 these two roots unite into a very short trunk in passing through the intervertebral 

 foramen ; and this mixed nerve divides almost immediately into two terminal 

 branche>^ — a superior, destined to the spinal muscles and the inteuuments 

 covering them ; the other, inferior, passes to the lateral and lower parts of the 

 trunk or to the limbs. All send, from their inferior branch, one or more 

 ramuscules to form the great sympathetic. 



The roots of the spinal nerves offer everywhere the same disposition ; each 

 nerve is t\)rmed by two fasciculi of converging filaments — superior and inferior — 

 which are naturally more nimierous and large when tiiey belong to voluminous 

 trunks, as may be remarked in those proceeding from the two bulbous enlarge- 

 ments of the spinal cord. The common axis of these two fasciculi affects a 

 transverse direction in nearly all the spinal pairs : but that of the posterior 

 nerves inclines more backwards, as they reach the terminal extremity of the cord. 



The filaments of the superior fasciculi — or sensitive roots — are larger than the 

 others, and emerge from the collateral groove on the spinal cord. They may be 

 followed, in animals, to the cells of the superior grey cornua. But these cells are 

 not sufficiently numerous to receive all the fibres of the superior roots, and some 

 of these pass into the cells of Clarke's columns. 



The filaments of the inferior fasciculi — or motor roots — arise opposite the pre- 

 ceding, on the lower face of the cord, at a short distance from the middle line, 

 and on the limits of the inferior and lateral columns. They may also be traced 

 to the interior of the cord, as far as the inferior grey cornua. 



These filaments do not unite to form their common trunk until after they 

 have passed through the dura mater. This is an extremely short trunk that 

 occupies the corresponding intervertebral foramen, and presents on its upper 

 face a ganglionic enlargement, which is exclusively placed on the course of its 

 sensitive fibres : the motor filaments being simply laid beside them, and do not 

 mix with them until beyond the ganglion. Immediately after this union, a small 

 filament is given off that enters the spinal canal to be distributed to the sinuses 

 and the bodies of the vertebrae. 



After leaving the meningeal sheath, the radicular fasciculi of the nerves, 

 furnished by the terminal extremity of the spin-al cord, run a somewhat long 

 course in the sacral canal, before finally uniting and passing into the tissues. 

 The common fasciculus they collectively form at the posterior extremity of the 

 spinal canal is named the caada equina. 



The distributive branches of the spinal nerves cannot be considered in a 



